John Gill Commentary Joshua 6:18

"But as for you, only keep yourselves from the devoted thing, lest when ye have devoted it, ye take of the devoted thing; so would ye make the camp of Israel accursed, and trouble it." — Joshua 6:18 (ASV)

And you in any wise keep [yourselves] from the accursed
thing
From laying hold on, secreting, and enjoying as their own, what was devoted to another use:

lest you make [yourselves] accursed ;
or a "cherem", also devoted to destruction:

when you take of the accursed thing ;
any part of it, be it what it will, gold or any other metal, or raiment, and the like:

and make the camp of Israel a curse, and trouble it ;
for being done secretly, and not known who did it, the whole body of the people would be chargeable with it, and suffer on account of it, unless discovered and punished, and as more fully appears by an after instance.